Netlify build started failing with no new commits/code changes

Usually, a “build that used to work but doesn’t any more” relates to the fact that you’re building without a cache. Previously, we may have cached node_modules which were imperative to your site building. But, without the cache (usually dropped after a month without a build, or on request) you may encounter errors.

Does your site build locally? Have you ensured that your dependencies are met? Perhaps mtg-shelf is a private module?